log: use native journald protocol for when running as a service

When running waybar as a systemd service, allows a slighty more readable
and searchable (in particular, by log level) using journal metadata.

meson.build

@@ -111,6 +111,10 @@ gtk_layer_shell = dependency('gtk-layer-shell-0', version: ['>=0.9.0'],
         default_options: ['introspection=false', 'vapi=false'],
         fallback: ['gtk-layer-shell', 'gtk_layer_shell'])
 systemd = dependency('systemd', required: get_option('systemd'))
+libsystemd = dependency('libsystemd', required: get_option('systemd'))
+if libsystemd.found()
+    add_project_arguments('-DHAVE_LIBSYSTEMD', language: 'cpp')
+endif
 
 cpp_lib_chrono = compiler.compute_int('__cpp_lib_chrono', prefix : '#include <chrono>')
 have_chrono_timezones = cpp_lib_chrono >= 201611
@@ -541,6 +545,7 @@ executable(
         upower_glib,
         pipewire,
         playerctl,
+        libsystemd,
         libpulse,
         libjack,
         libwireplumber,

src/main.cpp

@@ -7,6 +7,17 @@
 #include <list>
 #include <mutex>
 
+#ifdef HAVE_LIBSYSTEMD
+#include <spdlog/sinks/systemd_sink.h>
+#include <sys/stat.h>
+
+#include <cassert>
+#include <charconv>
+#include <cstddef>
+#include <cstdlib>
+#include <system_error>
+#endif
+
 #include "bar.hpp"
 #include "client.hpp"
 #include "util/SafeSignal.hpp"
@@ -154,7 +165,48 @@ static void handleSignalMainThread(int signum, bool& reload) {
   }
 }
 
+static void logToJournalIfRunAsService() {
+#ifdef HAVE_LIBSYSTEMD
+  /* Implementation of automatic protocol upgrading (from stderr to journal)
+  ** as described in https://systemd.io/JOURNAL_NATIVE_PROTOCOL */
+  char const* journal_stream = std::getenv("JOURNAL_STREAM");
+
+  if (journal_stream != nullptr) {
+    dev_t device;
+    ino_t inode;
+    size_t len = std::strlen(journal_stream);
+
+    auto result = std::from_chars(journal_stream, journal_stream + len, device);
+    if (result.ec == std::errc{})
+      result = std::from_chars(result.ptr + 1, journal_stream + len, inode);
+    if (result.ec != std::errc{}) {
+      spdlog::warn("malformed JOURNAL_STREAM (\"{}\"): {}, logging to console", journal_stream,
+                   std::make_error_condition(result.ec).message());
+    }
+
+    struct stat f_stderr;
+
+    if (fstat(STDERR_FILENO, &f_stderr) != 0) {
+      spdlog::warn("unable to check stderr device and inode numbers: {}", strerror(errno));
+    } else if (device == f_stderr.st_dev && inode == f_stderr.st_ino) {
+      auto journald = spdlog::systemd_logger_st("native_journal", "waybar", false);
+      /* systemd_logger_st is thread-safe with enable_formatter = false
+      ** thanks to underlying sd_journal_send being thread-safe
+      ** https://github.com/gabime/spdlog/issues/2320#issuecomment-1079766037
+      */
+      spdlog::set_default_logger(journald);
+    } else {
+      spdlog::info("JOURNAL_STREAM does not point to stderr, logging to console");
+    }
+  } else {
+    spdlog::info("no JOURNAL_STREAM, logging to console");
+  }
+#endif
+}
+
 int main(int argc, char* argv[]) {
+  logToJournalIfRunAsService();
+
   try {
     auto* client = waybar::Client::inst();